Two of the major determinants of renal vascular tone and responsiveness in normal man - age and salt intake - are under investigation. Assessment is made by renal arteriography and radioxenon washout. Responses to vasodilators (acetylcholine, dopamine and phentolamine) and vasoconstrictor agents (norepinephrine and angiotensin) are used to define reactivity in normal subjects and in patients with essential and secondary hypertension. In normal man flow/g falls with increasing age with an attendant decrease in the response to vasodilators. The results suggest the development of a "collar." Vasoconstrictor responses, conversely, do not change with age. Thus either the ratio of vascular lumen to effective wall thickness is unmodified, or - more likely - the effect of the smooth muscle atrophy is offset by the increased mechanical efficiency. Salt restriction does not influence responses to vasodilators, but reduces by an order magnitude the renal vascular response to angiotensin. The reduced response to angiotensin is specific: norepinephrine-induced vasoconstriction is somewhat potentiated.